The present invention relates to a coke oven having improved combustion chambers and a method of operating the same. The coke oven of the present invention allows uniform combustion to be achieved in the direction of the height of the combustion chambers, thereby reducing the Nox (nitrogen oxides) in the waste gas generated as a result of combustion.
The basic performance required of a coke oven is to produce high-quality coke, to reduce fuel consumption, and to achieve these objects at a low cost. In addition to such basic performance, what has been called for in recent years is less Nox contents in the waste gas.
Requirements for preventing environmental pollution have become increasingly severe year by year. The regulated NOx emission standards, specified by law for newly installed coke ovens, are quite stringent compared with those for existing coke ovens, and thus it is likely that new coke ovens cannot be constructed based on the prior art.
The NOx content in the waste gas increases with increasing combustion temperature. Therefore, Nox in the waste gas of a coke oven can be reduced by decreasing the combustion temperature in the combustion chambers. However, the combustion temperature must be higher than a predetermined value for the purpose of producing coke, and inevitably increases with higher operation rates. Therefore, the most realistic NOx reduction measure would be to eliminate localized, abnormal high temperature by achieving uniform combustion in the direction of oven height of the combustion chambers. However, since each combustion chamber of a coke oven has a slender, grooved structure (i.e., it is remarkably high in the vertical direction with respect to its horizontal cross-sectional area), it is difficult to achieve uniform combustion due to it""s a structure. The difficulty increases particularly with tall coke ovens.
The combustion temperature can be reduced locally by increasing flame lengths, e.g., by decreasing the calorific value of a fuel gas while diluting the fuel gas with the waste gas. The following methods are available as the specific measures:
(1) A method in which the waste gas in the combustion chamber is circulated, thereby increasing flame lengths and hence decreasing flame temperatures. This method is accomplished in Koppers circulation type coke ovens;
(2) A method in which combustion is scattered by partially supplying both the combustion air and a lean gas or only combustion air from a plurality of heightwise arranged ports partially (see Japanese Unexamined Patent Application Laid-Open Nos. 61-133286(1986) and 1-306494(1989), and Published Japanese Translation of PCT International Publication No. 4-501876(1992). This method is adopted in Carl Still coke ovens, Otto coke ovens, and Nippon Steel Corporation coke ovens as a multistage supply system for only combustion air, particularly when a rich gas in used as fuel. This method is called xe2x80x9cthe multistage combustion method.xe2x80x9d
Here, fuel gases used for coke ovens include not only a high calorific gas, such as a coke-oven gas called a rich gas, but also a gas called a lean gas. The rich gas means a fuel gas whose calorific value ranges from 14700 to 20160 kJ/Nm3 (3500 to 4800 kcal/Nm3), and the lean gas means a blast-furnace gas or a mixed gas of a blast-furnace gas and a coke-oven gas whose calorific values range from 3360 to 5460 kJ/Nm3 (800-1300 kcal/Nm3).
Therefore, (a) rich gas combustion and (b) lean gas combustion take place in a coke oven. An oven that can handle either (a) or (b) is called a single combustion coke oven, and an oven that can handle both (a) and (b) is called a compound combustion coke oven.
The method in (1), described previously, is aimed at accomplishing the slowing down of the combustion progress in the direction of oven height by reducing the oxygen content and the calorific value of the fuel gas while circulating the waste gas, and thus is effective for controlling the amount of NOx generated. However, in this method the amount of waste gas increases, and energy losses also increase when the amount of circulated waste gas is increased. Further, in the waste gas circulation method based on the Koppers coke oven circulation system, it is difficult to increase the waste gas circulation rate greatly due to the restricted cross-sectional area of a circulation port. The rate can be increased to about 20% at most. In addition, the amount of waste gas circulated cannot be varied as desired, either.
The method of reducing NOx by multistage combustion in (2), described previously, requires adjustment of the distribution ratio of the combustion air or lean gas, in the direction of oven height during the operation of the coke oven when the amount of fuel gas is greatly varied. However, in the actual coke oven operation, not only such an adjustment entails much time, but also the place to be adjusted is limited mainly to the ports at the uppermost stage and at the bottom, imposing difficulty adjusting the apertures of intermediate ports, and thus a satisfactory effect on NOx reduction cannot be obtained.
An exemplary bottom structure of the combustion chamber of a coke oven is disclosed in the previously described Published Japanese Translation of PCT International Publication No. 4-501876(1992) and Cokemaking International, Vol. 4-2, pp.71-83 (1992). As shown in FIG. 9(a), a rich-gas port 2 is arranged near an oven wall 6 of a coke oven, and a lean-gas port 7 and an air port 3 are arranged side by side in the middle. Further, Japanese Publication of Examined Patent Application No. 5-29678 (1993) discloses a drawing in which a lean-gas port and an air port extend in the direction of coke pushing (i.e., direction of oven length), side by side, almost in the middle of a combustion chamber. However, no description is made as to the arrangement and structure of the lean-gas port and the air port, for achieving a uniform combustion temperature in the direction of oven height and for reducing NOx in the waste gas.
The basic object of the present invention is to provide a coke oven and a method of operating the same that forms a waste gas containing less NOx.
A specific object of the present invention is to provide a coke oven, having a combustion chamber that can eliminate localized high-temperature combustion by achieving uniform combustion in the direction of oven height, even if the oven is of a tall, large-sized structure.
Another specific object is to provide a coke oven having a combustion chamber that can achieve the above-described uniform combustion independently of the combustion type, i.e., either single combustion in which either a rich gas or lean gas is used as fuel, or compound combustion in which both are used alternately.
Still another specific object is to provide a method of in operating a coke oven that allows NOx in the waste gas to be reduced by achieving uniform combustion within the combustion chamber.
The present invention pertains to a coke oven such as shown in FIG. 1. In FIG. 1, reference numerals I, II, III . . . denote arrays of combustion chambers, and i, ii, . . . , denote carbonization chambers. The combustion chamber arrays and the carbonization chambers are arranged alternately in the direction of oven battery (Y direction). Each combustion chamber array consists of many combustion chambers 1-1, 1-2, 1-3, 1-4 . . . that extend in the direction of coke pushing (X direction). What is to be improved by the present invention are the structure of these combustion chambers and the combustion method applied to such combustion chambers.
Here, the direction of oven battery means the direct ion in which many combustion chambers (specifically, a plurality of combustion chambers divided by flue partition walls, or so-called an array of flues) and carbonization chambers extend alternately in parallel. Further, the direction of coke pushing means the direction at right angles to the direction of oven battery, and in the direction of connecting the pusher side to the coke discharging side in a coke oven.
A method of combustion within the combustion chamber includes the singlestage combustion system, in which all of a rich gas or lean gas as a fuel gas is supplied from the bottom of combustion chamber and all of combustion air (hereinafter described simply as xe2x80x9cairxe2x80x9d) is supplied from the bottom of combustion chamber, and the multistage combustion system, in which part of air and/or a lean gas is supplied from the bottom and the rest thereof from one or a plurality of places in the direction of oven height. Furthermore, types of ovens include a single combustion oven for supplying only a rich gas or lean gas as a fuel gas, and a compound combustion oven that can supply a rich gas and a lean gas alternately. The present invention is directed to a coke oven having a structure capable of accommodating all these types of combustion systems.
A coke oven of the present invention comprises a combustion chamber having characteristic features (1) and (2) described below;
(1) As shown in FIG. 3, the rich-gas port 2 is located at the bottom 5 of combustion chamber near the oven wall 6 bordering the carbonization chamber;
(2) The midpoint P1 connecting the centers P of the air ports 3 at the bottom 5 is on the side opposite to the rich-gas port 2 across the center line CL extending in the direction of coke pushing of the combustion chamber in parallel to the oven wall 6.
It is further required that the following characteristic feature (3) and (4) be obtained for compound combustion.
(3) As shown in FIG. 6(a), when viewed in the direction of coke pushing (X direction) and in the direction of oven battery (Y direction) of the combustion chamber, the lean-gas port 7 and the air port 3, that have their openings in the bottom of the combustion chamber, do not completely overlap in any of these directions.
(4) As shown in FIG. 4, the midpoint P3 connecting the center P2 of the lean-gas port 7 and the center P of the air port 3 at the bottom is on the side opposite to the rich-gas port 2 across the center line CL.
Combustion chamber zones 5-1 and 5-2 defined by center line CL as shown in FIG. 3(b) are referred to the first zone and the second zone respectively hereinafter.
The lean-gas port 7 and the air port 3, described above, may partially overlap when viewed in the direction of coke pushing (X direction) or in the direction of oven battery (Y direction), as shown in FIGS. 7(b) and (c). At this time, it is desirable that the length of the overlapped openings is 80% or less of a complete overlapped length (L shown in FIG. 8). Further, an aperture adjusting member 9 may be attached to at least one of the lean-gas port 7 and the air port 3, as shown in FIG. 5, to thereby narrow the original overlap rate from Y2 to Y21, so the above-described overlap rate of 80% or less can be achieved. Here, the opening of a port means an opening originally provided when the oven was installed, or an opening narrowed by attaching the aperture adjusting member.
Methods of the present invention are operating methods of the above-described coke oven of the present invention, and the typical methods are as follows:
(1) A method of effecting singlestage combustion by supplying the total amount of a lean gas and that of air from ports at the bottom of combustion chamber, respectively;
(2) A method of effecting multistage combustion by supplying all of the lean gas from the port at the bottom of combustion chamber and part of the air (20 to 70% by volume) from the port at the bottom of combustion chamber, and supplying the rest of the air from one or more ports provided in a flue partition wall;
(3) A method of effecting multistage combustion by supplying part of the lean gas from the port at the bottom of combustion chamber and the rest thereof from the ports provided in the flue partition wall, and supplying all of the air from the port at the bottom of combustion chamber;
(4) A method of effecting multistage combustion by supplying part of the lean gas from the port at the bottom of combustion chamber and the rest thereof from the port(s) provided in the flue partition wall, and supplying part of the air (20-70% by volume) from the port at the bottom of combustion chamber and the rest thereof form the port(s) provided in the flue partition wall;
(5) A method of effecting singlestage combustion by supplying the total amount of a rich gas and that of the air from the ports at the bottom of combustion chamber;
(6) A method of effecting multistage combustion by supplying all of the rich gas from the port at the bottom of combustion chamber, and supplying part of the air (50% by volume or more) from the port at the bottom of combustion chamber and the rest thereof from the port(s) provided in the flue partition wall.
In any of the above-described methods, the air purging direction is changed by mounting the aperture adjusting member 9 on the opening of the lean-gas port 7 and/or the air port 3 that extends toward the rich-gas port 2, by crossing the center line CL extending in the direction of coke pushing of the bottom of combustion chamber, so that the mixing point of the rich gas and air can be adjusted. This adjustment has the function of changing the air purging direction oppositely to the rich-gas port. Further, in the case of lean gas combustion, it is desirable that not only the aperture adjusting members be mounted on the openings of the ports, to thereby obtain an overlap rate of 80% or less, but also the lean gas purging direction and the air purging direction be changed to adjust the mixing point of the lean gas and air. Here, xe2x80x9cthe mixing point of the rich gas or lean gas and airxe2x80x9d means the position in the direction of oven height from the bottom of combustion chamber at which the fluxes of the purged fuel gas and air initially collide with each other.